1. Field of the Invention
The present invention relates to methods for driving light-emitting diodes, light-emitting diodes, methods for driving displays, displays, methods for driving electronic devices, electronic devices, methods for driving optical communication apparatuses, and optical communication apparatuses. In particular, the present invention relates to methods for driving light-emitting diodes suitable for applications such as a variety of displays including InGaN/GaN-based light-emitting diodes.
2. Description of the Related Art
InGaN/GaN-based light-emitting diodes, particularly ones with emission wavelengths in the visible region, can vary in emission wavelength (color) with drive current density. Japanese Unexamined Patent Application Publication No. 2002-237619, for example, proposes a method for driving light-emitting diodes using this phenomenon to achieve polychromatic light emission.
When used for typical color displays, however, such light-emitting diodes cause the problem that a desired color range where the color varies with drive current density is difficult to provide. It may also be possible to convert chromaticity according to colors and color ranges varying with current density. This method, however, results in a narrower color range than normal because only a common portion of varying color ranges is used for accurate color reproduction. In addition, the need for enormous quantities of calculation, depending on the number of pixels and frame rate, puts a high load on signal processing circuits and drive circuits.
To avoid this phenomenon when light-emitting diodes are used for displays, for example, Japanese Unexamined Patent Application Publication No. 2003-22052 discloses a technique for generating luminance signals by pulse driving (such as pulse density modulation (PDM) or pulse width modulation (PWM)) while maintaining constant current density to prevent color variations. Also, Japanese Unexamined Patent Application Publication No. 2005-260116 discloses a technique for modulating luminance only by pulse driving while controlling current density so as to provide a desired emission wavelength using the phenomenon that the color varies with current density. These techniques are effective when light-emitting diodes, which can have color variations after manufacture, are applied to high-quality displays. However, the need for adjustment of emission wavelength and luminance contributes to increased inspection/adjustment costs. In addition, these techniques put a high load on signal processing circuits and drive circuits. Furthermore, extending the dynamic range of luminance requires the use of a high-frequency circuit to increase pulse frequency.
U.S. Pat. No. 6,635,904, for example, proposes the production of InGaN/GaN-based light-emitting diodes using a vicinal substrate as a growth substrate, for example, a sapphire substrate having a main surface inclined at an angle of 0.2° to 2° with respect to the c-plane. According to this publication, the light-emitting diodes have improved luminous efficiency and homology. This publication, however, has no disclosure or suggestion as to the current density dependence of the emission wavelength of the light-emitting diodes or no description of advantages for application to displays.